The present invention generally relates to a III-V compound semiconductor laser device. Particularly, it relates to the structure of an AlGaInP semiconductor laser device, which can be operated at a low voltage, and a process for producing the same.
The AlGaInP semiconductor material having a lattice constant almost equal to a lattice constant of a GaAs substrate can achieve crystal growth with high quality. Further, because the AlGaInP semiconductor material is a direct transition-type semiconductor having the largest bandgap among the III-V compound semiconductor materials other than nitrides, it has been developed as a light-emitting material for light in the visible range. In particular, AlGaInP semiconductor laser devices have been widely used as light sources for optical disks, because they have shorter oscillation wavelengths compared with AlGaAs semiconductor laser devices, and enable a high-density recording.
The bandgap of the (AlxGa1-x)yIn1-yP (0xe2x89xa6xxe2x89xa61, y=0.5) material can be varied between 1.91 eV (GaInP) and 2.35 eV (AlInP) by changing the mixed crystal ratio x of Al from 0 to 1. Incidentally, the bandgap of GaAs is about 1.42 eV, and there is a big difference in the bandgap between a GaAs material and an AlGaInP material. When an AlGaInP layer is grown on a GaAs layer, band discontinuity due to a big difference in the bandgap occurs at a hetero-interface between the two layers. In particular, large band discontinuity occurs in a valence band and it acts as a barrier against injected holes resulting in an increase in the operation voltage of the laser element.
It is known from JP-A-5-7049, for example, that the above problem can be solved by providing, between a GaAs layer and an AlGaInP layer, a layer having a bandgap intermediate between the two layers. FIG. 9 is a view seen from an end surface of a semiconductor laser device taking such a countermeasure. Referring to FIG. 9, an n-type GaAs buffer layer 102, an n-type GaInP intermediate layer 103, an n-type AlInP cladding layer 104, a GaInP active layer 105, a p-type AlInP cladding layer 106, a GaInP etch stop layer 107, a p-type AlInP second cladding layer 108, a p-type GaInP intermediate layer 109, and a p-type GaAs contact layer 110 are formed in order on an n-type GaAs substrate 101 using an MBE method. Then the p-type GaAs contact layer 110, the p-type GaInP intermediate layer 109 and the p-type AlInP second cladding layer 108 are removed by etching, excluding a stripe-geometry ridge portion 120. Subsequently, an n-type GaAs current block layer 111 is formed at portions other than the stripe-geometry ridge portion 120, thereby obtaining a crystals-stacked structure called a wafer. After that, an n-type electrode 112, and a p-type electrode 113 are deposited, and the wafer is divided into bar-shaped pieces. A protective film is formed on each end surface of the resulting bars, thereafter the bars are divided into chips serving as semiconductor laser devices.
The p-type AlGaInP second cladding layer 108 is doped with beryllium (Be) as an impurity to a density of 4xc3x971017 cmxe2x88x923. Similarly, the p-type GaInP intermediate layer 109 is doped with Be to a density of 1xc3x971019 cmxe2x88x923 as an impurity, and the p-type GaAs contact layer 110 is doped with Be to a density of 5xc3x971018 cmxe2x88x923 as an impurity.
In the above structure, between the p-type AlInP second cladding layer 108 having a large bandgap and the p-type GaAs contact layer 110 having a small bandgap, the p-type GaInP intermediate layer 109, which has a bandgap intermediate between the above two layers, is provided, whereby band discontinuity at the interface is reduced. In addition to that, as the density of the p-type GaInP intermediate layer 109 having an intermediate bandgap increases, the band discontinuity is reduced.
The inventors have determined, with respect to the above prior art example, that if the impurity density exceeds a certain level (for example, 7xc3x971019 cmxe2x88x923 for GaInP crystals), impurity atoms do not enter appropriate lattice sites and become lattice defects such as interstitial atoms, which bring about deterioration of the quality of crystals. Therefore, the impurity density that can be doped for improving the band discontinuity has an upper limit.
Further, in the prior-art example, the p-type GaInP intermediate layer 109 is doped with Be to a density of 1xc3x971019 cmxe2x88x923, which is in the range that would not deteriorate the quality of crystals. Yet, of laser elements obtained according to the prior art, some laser elements had a high operation voltage. However, the inventors have determined that operation voltage of higher than 2.3 V does not allow practical reliability to be attained.
As a reason for this, it is presumed as follows: when the p-type GaInP intermediate layer 109 is formed and the p-type GaAs contact layer 110 is formed thereon, or when the n-type GaAs current block layer 111 is grown after forming the stripe-geometry ridge region 120, the wafer having the p-type GaInP intermediate layer 109 doped with impurities to a high density and the p-type GaAs contact layer 110 is retained at a high temperature, and thus Be atoms are diffused from the p-type GaInP intermediate layer 109 to the p-type GaAs contact layer 110. Such diffusion of impurities, which is sensitive to the temperature, and which strongly depends on the in-plane temperature distribution of the wafer, is an unstable phenomenon. For that reason, when operating the semiconductor laser device, the Be impurity density of the p-type GaInP intermediate layer 109 has already been reduced or varied and therefore the band discontinuity at the interface is not reduced sufficiently. As a result, the resistance of the laser element increases and heat generation of the laser element increases.
Under the circumstances, it has been desired to realize semiconductor laser devices that maintain the doping density of a layer having an intermediate bandgap even after a wafer was retained in a high-temperature state as in the crystal growth, that operate at a low voltage, and that have an operation voltage distribution in a narrow range.
The present invention was made in order to solve the above problem, and an object of the present invention is to provide a ridge stripe-type AlGaInP semiconductor laser device in which diffusion of impurities from a contact layer or an intermediate bandgap layer is suppressed so that the impurity density of the intermediate bandgap layer is maintained high enough and that the semiconductor laser device has a low operation voltage.
According to an aspect of the present invention, there is provided a semiconductor laser device comprising:
an active layer;
a first-conductivity type cladding layer and a second-conductivity type cladding layer sandwiching the active layer therebetween;
a second-conductivity type contact layer disposed above the second-conductivity type cladding layer and having a bandgap different from a bandgap of the second-conductivity type cladding layer; and
a second-conductivity type intermediate bandgap layer disposed between the second-conductivity type cladding layer and the second-conductivity type contact layer and having an intermediate bandgap between the bandgaps of the second-conductivity type cladding layer and the second-conductivity type contact layer,
wherein said second-conductivity type contact layer comprises at least a first contact layer, an intermediate second contact layer and a third contact layer stacked in this order and the second contact layer has an impurity density lower than impurity densities of the first and third contact layers.
This arrangement suppresses diffusion of impurities, so that the intermediate bandgap layer can exhibit the effect of reducing band discontinuity sufficiently. Also, This arrangement allows impurities to be prevented from being diffused to the active layer, whereby an increase in the oscillation threshold current is suppressed. Thus, favorable characteristics of the semiconductor laser device are obtained.
In one embodiment, the second-conductivity type cladding layer has an impurity density smaller than that of the intermediate bandgap layer, and the impurity density of the intermediate bandgap layer is equal to or smaller than that of the first contact layer.
With this arrangement, even if the wafer is retained at a high temperature, diffusion of impurities is suppressed, and the impurity density of the intermediate bandgap layer can be secured. Therefore, it is possible to reduce the band discontinuity at the interface and lower the operation voltage, which results in an improvement in the yield of semiconductor laser devices.
In one embodiment, the semiconductor laser device has a stripe-shaped region for injecting an electric current into the active layer and a first-conductivity type current block layer is provided in regions other than the current injection region.
With this arrangement, it becomes possible to control optical radiation characteristics, resulting in an improvement in the performance of the semiconductor laser device.
The second-conductivity type cladding layer may comprise a first cladding layer and a second cladding layer on or above the first cladding layer, and the stripe-shaped region may comprise at least the second cladding layer from among the second cladding layer, the intermediate bandgap layer and the second-conductivity type contact layer.
Each of the first-conductivity type cladding layer, the active layer, the second-conductivity type cladding layer and the intermediate bandgap layer may be made of (AlxGa1-x)yIn1-yP (0 less than x less than 1, 0 less than y less than 1). With this arrangement, it is possible to realize a semiconductor laser device with a light-emitting wavelength in a 600-nm band, which is adapted to a system such as DVD and DVD-RW.
The second-conductivity type contact layer may be made of GaAs. With this arrangement, it becomes easy to form an ohmic contact of the contact layer with an electrode metal. As a result, this structure is effective in reducing the operation voltage of the semiconductor laser device.
In one embodiment, the second-conductivity type is a p type, and the second-conductivity type cladding layer, the second-conductivity type intermediate bandgap layer and the second-conductivity type contact layer contain beryllium (Be) as a p-type impurity. Be is a p-type impurity that can be doped in an AlGaInP-type material up to a high density. Further, diffusion of Be in crystals is slighter than zinc (Zn), which is used as a p-type impurity in the MOCVD (organic metal vapor deposition) method. Therefore, it is possible to realize an impurity profile as designed. Moreover, the impurity profile has superior reproducibility, which can greatly contribute to an improvement in the characteristics and the yield of semiconductor laser devices.
In one embodiment, the intermediate bandgap layer has an impurity density of 5xc3x971018 cmxe2x88x923 or higher. With this arrangement, it becomes possible to hold down the operation voltage of the semiconductor element.
Additionally or alternatively, the second contact layer may have an impurity density in the range between 5xc3x971017 cmxe2x88x923 and 5xc3x971018 cmxe2x88x923 inclusive. With this arrangement, the operation voltage of the semiconductor laser device is lowered and also it becomes possible to suppress an increase in the oscillation threshold current.
Additionally or alternatively, the third contact layer may have an impurity density of 5xc3x971018 cmxe2x88x923 or higher. With this arrangement, a favorable ohmic contact of the third contact layer with an electrode metal can be realized and the operation voltage can be lowered.
According to another aspect of the present invention, there is provided a process for producing a semiconductor laser device in which an active layer is sandwiched between first-conductivity type and second-conductivity type cladding layers, and a second-conductivity type contact layer is disposed above the second cladding layer with an intermediate bandgap layer interposed between the second cladding layer and the contact layer, the second-conductivity type contact layer having a bandgap different from a bandgap of the second-conductivity type cladding layer, the intermediate bandgap layer having an intermediate bandgap between the bandgaps of the second-conductivity type cladding layer and the second-conductivity type contact layer, the process comprising, after forming the active layer:
forming the second-conductivity type cladding layer by molecular beam epitaxial method;
forming the second-conductivity type intermediate bandgap layer on the second-conductivity type cladding layer by molecular beam epitaxial method; and
forming the second-conductivity type contact layer on the intermediate bandgap layer by molecular beam epitaxial method,
wherein the step of forming the second-conductivity type contact layer comprises forming at least a first contact layer, an intermediate second contact layer, and a third contact layer such that the second contact layer has an impurity density lower than impurity densities of the first and third contact layers.
The process may further comprise:
forming a stripe-shaped region for injecting an electric current into the active layer; and
forming a first-conductivity type current block layer in regions other than the current injection region by molecular beam epitaxial method.
Use of the molecular beam epitaxial (xe2x80x9cMBExe2x80x9d) method for the crystal growth of the stacked semiconductor structure allows Be to be used as a p-type impurity. Furthermore, the MBE method suppresses abnormal growth of crystals such as projection-like crystals in proximity of a ridge (the current injection region) when forming the current block layer, and realizes a smooth crystal surface with little unevenness. For that reason, the aftertreatment is easy, thus simplifying the processes. Further, when a laser chip is mounted on a heat radiation material in a junction-down manner, because the laser chip is superior in adhesion properties, the temperature characteristics of the semiconductor laser device are improved.
Other objects, features and advantages of the present invention will be apparent from the following description.